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Libreville, Gabon

Engone Obiang N.L.,IRET | Ngomanda A.,IRET | Hymas O.,University College London | Chezeauxl T.,Rougier Gabon | And 2 more authors.
Forest Ecology and Management | Year: 2014

The diameter distribution of a tree species population in a natural forest reflects its demographic dynamics and the impact of past disturbances. A modal diameter distribution can be the mark of a demographic imbalance or it can be the consequence of a particular growth pattern. By comparing the observed diameter distribution with the one that follows from a reference growth model under the hypothesis of demographic balance, we diagnosed demographic balance. In a forest concession in Gabon before logging, an unbalanced diameter distribution was found for Aucoumea klaineana, a long-lived pioneer species and also a valuable commercial species. The Aucoumea klaineana population consisted of old trees without meaningful recruitment, thus indicating an ageing, declining population. In contrast, in the same concession, a balanced diameter distribution was found for Lophira alata, a pioneer to moderate light-demanding species. The history of this forest concession, once heavily populated and subsequently depleted from human perturbation (shifting cultivation), can be used to interpret these demographic statuses. These results reinforce the view of a forest that is constantly changing and shaped by past human perturbations, with the implication for its conservation that some kind of perturbation should be maintained. © 2013 Elsevier B.V.

Ngomanda A.,IRET | Engone Obiang N.L.,IRET | Lebamba J.,Universite des Sciences et Techniques de Masuku | Moundounga Mavouroulou Q.,IRET | And 15 more authors.
Forest Ecology and Management | Year: 2014

Fewer data on tree biomass and allometric equations are available in African tropical moist forests than in the other tropical continents. When needing a biomass allometric equation, one thus faces the dilemma of using either a pantropical equation with the risk that it is biased for Africa, or a site-specific equation that is imprecise. Using a data set on aboveground biomass for 101 trees destructively measured at Zadié in northeastern Gabon, we fitted site-specific allometric equations and assessed the validity of ten existing equations. The best fitted model without height as a predictor was: B = exp[ - 4.0596 + 4.0624lnD - 0.228(lnD)2 + 1.4307lnρ], whereas the best fitted model with height was: B = exp[ - 2.5680 + 0.9517ln(D2H) + 1.1891lnρ], where B is the aboveground biomass in kg, D the diameter at breast height in cm, H the height in m, and ρ the wood density in gcm-3. Separate allometric equations for the stem, stump, foliage and branches were also fitted. Chave et al. (2005)'s pantropical equations for moist forests, that are currently the most commonly used allometric equations in central Africa, were not valid at Zadié with an overestimation of biomass of about 40%. The allometric equations of the same authors for wet forests were valid at Zadié, even though the climatic zone does not correspond. More data on tree biomass are needed in central Africa to explore the natural range of variability in tree biomass and identify the factors that influence variations among sites. © 2013 Elsevier B.V.

Chave J.,CNRS Biological Evolution and Diversity Laboratory | Rejou-Mechain M.,CNRS Biological Evolution and Diversity Laboratory | Burquez A.,National Autonomous University of Mexico | Chidumayo E.,Makeni Savanna Research Project | And 20 more authors.
Global Change Biology | Year: 2014

Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven-dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees = 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as covariates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter-height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability, and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter-height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development. © 2014 John Wiley & Sons Ltd.

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